In recent years, there has been a sharply increasing demand for high energy density, non-aqueous rechargeable batteries as portable electronic equipment such as cell phones and note type personal computers have developed and electric automobiles have become commercially available. Today, non-aqueous rechargeable batteries that could satisfy the demand use a wide variety of positive electrode active materials including lithium cobalt oxides such as lithium cobaltate (LiCoO2), lithium nickel composite oxides such as lithium nickelate (LiNiO2) and lithium manganese composite oxides such as lithium manganite (LiMn2O4).
Among the above, lithium nickelate having a greater discharge capacity than lithium cobaltate has been expected as a positive electrode active material that could form inexpensive high energy density batteries with reduced use of cobalt whose reserves are limited.
However, lithium nickelate has a less stable crystal structure than that of lithium cobaltate in a charged state and cannot be used to make sufficiently safe batteries as it is. In addition, as for a charge/discharge cycle life, the batteries produced using lithium nickelate do not provide satisfactory properties because of the low reversibility of the crystal structure of lithium nickelate.
In view of the circumstances, the use of lithium nickel composite oxides having a part of Ni substituted by an element such as Co, Al and Y has been proposed in order to maintain the crystal structure of lithium nickelate in a charged state, and there have been attempts to improve the safety and reversibility (see for example Patent Document 1).
However, when a positive electrode mixture that contains the lithium nickel composite oxide described above, a conduction aid, and a binder is dispersed in a solvent and prepared into a slurry or paste type positive electrode mixture containing composition, and the composition is applied on one or both surfaces of a collector made of a metal foil, followed by drying to form a positive electrode mixture layer, gelation of the positive electrode mixture containing composition is more likely to proceed, which results in a loss in the productivity of the positive electrode and hence a loss in the productivity of the non-aqueous rechargeable battery since the pot-life of the positive electrode mixture containing composition during producing the positive electrode is short.
The positive electrode active material is prone to adsorb moisture, and the adsorbed moisture must be removed for example by vacuum drying before the material is used, or otherwise a gas generated by a reaction with an electrolytic solution could make a battery swell during storage. This could significantly reduce the charge/discharge cycle characteristic of the battery in some cases.
In order to prevent the above-described problems related to the lithium containing composite oxide, it has been proposed to treat a surface of a positive electrode active material with a silane coupling agent (Patent Document 2). However, it has been found based on the inventors' studies that properties or the like of a lithium containing composite oxide in treatment greatly change the effects.